Radiation is commonly used in the non-invasive inspection of contents of objects, such as luggage, bags, briefcases, cargo containers, and the like, to identify hidden contraband at airports, seaports, and public buildings, for example. The contraband may include hidden guns, knives, explosive devices, illegal drugs, and Special Nuclear Material, such as uranium and plutonium, for example. One common inspection system is a line scanner, where the object to be inspected is passed through a fan beam or pencil beam of radiation emitted by a source of X-ray radiation. Radiation transmitted through the object is attenuated to varying degrees by the contents of the object and detected by a detector array. Attenuation is a function of the type and amount (thickness) of the materials through which the radiation beam passes. Radiographic images of the contents of the object may be generated for inspection, showing the shape, size and varying amounts of the contents. In some cases the material type may be deduced.
The inspection of cargo containers at national borders, seaports, and airports is a critical problem in national security. Due to the high rate of arrival of such containers, 100% inspection requires rapid imaging of each container. Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters), and 6-9 feet wide (1.8-2.7 meters). Larger air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may be up to about 240×118×96 inches (6.1×3.0×2.4 meters). MeV radiation sources are typically required to generate radiation with sufficient energy to penetrate through standard cargo containers and the larger air cargo containers.
MeV radiation sources typically comprise a particle accelerator, such as a linear radiofrequency (“RF”) particle accelerator, to accelerate charged particles, and a source of charged particles, such as an electron gun, to inject charged particles into the accelerator. The linear accelerator may comprise a series of linearly arranged, electromagnetically coupled resonant cavities in which standing or traveling electromagnetic waves for accelerating the charged particles are supported. The charged particles injected into the resonant cavities are accelerated up to a desired energy and directed toward a conversion target to produce radiation Where the accelerated charged particles are electrons and the target is a heavy material, such as tungsten, Bremsstrahlung or X-ray radiation is generated. Electrons accelerated to a nominal energy of 6 MeV and impacting tungsten, will cause generation of X-ray radiation having an energy of 6 MV, for example.
A microwave (RF) power source provides RF power to the cavities of the accelerator. The microwave source may be an oscillating microwave power tube, such as a magnetron, or an amplifying microwave power tube, such as a klystron. The microwave sources are powered by modulators, which generate high electric power pulses having peak electric powers of from 1 MW to 10 MW, and average powers of from 1 kW to 40 kW, for example.
Characteristics of the modulator output may be varied to vary the output of the microwave power source. For example, the amplitude of the high-voltage pulses driving the oscillator or the amplifier may be varied to change the microwave power output. Alternatively, in an amplifier, the microwave input signal may be varied to change the microwave power output.
The accelerator, which may have a loaded Q value of 5000, for example, is very sensitive to the frequency of the input RF power. Maximum acceptance of microwave power provided by the RF source is achieved when the center frequency of the microwave power matches the accelerator resonance frequency. Otherwise, some or most of the microwave power provided to the accelerator will be reflected, preventing acceleration of the charged particles to the desired beam energy. The RF frequency may be adjusted to match the accelerator resonance frequency by a mechanical or electrical tuner.
The RF power provided to the accelerator causes heating and expansion of the accelerator structure, which causes a slow frequency drift of the accelerator resonance frequency. Such drift is most noticeable in the first minute or two of operation, but may continue due to environment conditions.
An automatic frequency controller (“AFC”) is generally required to servo the RF source frequency to track the accelerator resonance frequency, as is known in the art. The AFC samples and compares microwave signals provided to the accelerator with those reflected from the accelerator, to determine the required tuning of the microwave source. An AFC is generally sufficient to match the frequency of the RF source to the resonance frequency of the accelerator, during steady state operation. An example of an AFC is described in U.S. Pat. No. 3,820,035, which is incorporated by reference herein.
When a magnetron is used, pulse to pulse frequency jitter in the magnetron may also cause a small mismatch between the frequency of a magnetron and the resonance frequency of the accelerator. Such mismatch varies from pulse to pulse and adds some noise to the system. This may be improved by feeding some microwave power reflected from the accelerator back into the magnetron by a reflector and variable phase shifter, for example, as described in U.S. Pat. No. 3,714,592, which is also incorporated by reference herein. The reflector/variable phase shifter may be referred to as a “phase wand.”
It is difficult to distinguish nuclear devices and nuclear materials from other dense or thick items that may be contained within the object by standard X-ray scanning. The information that may be derived about the material type of the contents of objects by X-ray scanning may be enhanced by the use of radiation beams in the MV energy range, with two or more different energy spectra that interact differently with the material contents of the object. For example, the attenuation of a 6 MV X-ray radiation beam by the contents of the object will be different from the attenuation a 9 MV X-ray radiation beam by the same contents, due to the differing effects of Compton Scattering and induced pair production on the different energy beams. A ratio of the attenuations at the two X-ray energies may be indicative of the atomic numbers of the material through which the radiation beam passes, as described in U.S. Pat. No. 5,524,133, for example. More sophisticated dual energy analysis techniques are described in U.S. Pat. No. 7,257,188, for example, which is assigned to the assignee of the present invention and incorporated by reference herein. Ratios of high and low energy attenuations may also be plotted against object thickness to facilitate material identification, as described in “Dual Energy X-ray radiography for automatic high-Z material detection,” G. Chen et al, NIM (B), Volume 261 (2007), pp. 356-359.
It would be useful to be able to generate radiation beams having different nominal energies in the MV range by a single radiation source for the dual energy inspection of cargo containers and other objects, for example. In an example of an interlaced dual energy accelerator described in U.S. Pat. No. 7,130,371 B2, different electron beam energies are achieved in a traveling wave accelerator by changing the electron beam loading and RF frequency of the accelerator in a synchronized manner and thereby changing the effectiveness of acceleration. No successful reports of field application of this approach are known, possibly due to the complexity of the system and stability issues.